1. Field of the Invention
This invention relates to a semiconductor diode laser array.
2. Description of the Related Art
The single element laser diode is an extremely attractive optical source due to the large number of advantages it offers. These advantages can include small size, rugged construction, high efficiency, direct current pumping capability and a potential for low manufacturing cost and integrability. The output power available from such a diode, however, is somewhat limited. If the output power could be improved it would enable the diode to be used in new applications where it could replace types of laser source that may lack some of these advantages.
Conventional laser diodes have active regions with a lateral width of a few microns. This ensures that the laser operates stably in a single spatial lateral optical mode. It might be possible to increase the potential optical power by having a wider active region but this usually leads to an unstable and filamentary lasing action with poor beam quality.
The resulting uneven distribution of the optical power also reduces the efficiency, and the high power capability is limited because local areas of high peak power can be present.
In attempts to increase the output power, various devices having more than one active stripe have been developed. If the distance between the array elements is large compared with the spot size for the individual elements there will be no coupling between the elements. This means that the output power will add incoherently and consequently the beam divergence will be identical to that of a single element.
To overcome these problems, the active stripes could be brought close together and phase-locked operation could be achieved. By these means, structures with a large number of active stripes have shown a power output of up to 2.6 watts and in a device with several groups of coupled stripes a power output of eleven watts has been achieved.
However, it is often observed that the beam quality of a laser array is significently inferior to that which would be expected from a coherent addition of the individual beams. Frequently, the far field consists of two lobes. The reason for this behaviour is that in many simple array structures there is no lateral waveguiding. The waveguiding is instead induced by the charge carriers, this is often called "gain guiding". Since there is a nonlinear interaction between the carrier density profile and the intensity distribution, spatial stability problems are very likely to occur. It appears now to be generally agreed that in order to achieve a single lobed diffraction limited far field it is necessary that some degree of lateral built-in waveguiding for each array element should be present. This may be achieved in a number of different ways.
It is observed that at high power even an index guided array may not have the desired single lobed far field. Instead, the array operates in a higher order mode in which adjacent array elements are 180.degree. out of phase, which gives rise to a twin lobed far field. In order to improve the beam quality it has been suggested that improved performance may be possible if there is a sufficient optical gain between the array elements.
Another solution to the problem is to provide an array design with a high degree of uniformity as described in copending United Kingdom Patent Application Ser. No. 8628368, now published as application No. GB 2196788A, assigned to the same assignee as the present application. This discloses a laser array device having a spaced arrangement of laser elements positioned such that the optical field in one element partly overlaps that of a neighbouring element, in which the sum of the propagation constant and the coupling parameters to the neighbour elements equals the same value for all elements in the array. The propagation constant can be defined as the angular frequency of the light divided by the speed of light in the structure. The coupling parameter is a measure of the degree of overlap of the optical field in one element with that in a neighbouring element.
An alternative solution to the problem of providing a higher power output would be to use a surface emitting laser diode. Some examples of this construction use a tilted deflector or a grating to couple the optical power. In the last-mentioned construction, the emission occurs from a passive section, with the active region being placed away from the output coupling region. This implies that the output coupling occurs over a distance of the order of the coupling length of the grating, and the output power is only marginally increased by making the coupling region longer than the coupling length.